The invention is based on a priority application EP 01 440 315.8 which is hereby incorporated by reference.
The present invention relates to a CS-RZ optical clock signal generator having a high clock frequency, and to any optical device incorporating such a generator, such as a resynchronized optical multiplexer, for example. The invention applies in particular to optical transmission systems operating at very high bit rates.
In conventional manner, the power spectrum density of a return-to-zero (RZ) optical signal is relatively broad because of the large number of transitions in the signal to be transmitted. Because the transmitted energy is thus spread over a wide range of frequencies, an RZ signal is sensitive to group velocity dispersion (GVD) i.e. to chromatic dispersion, and also to four-wave mixing (FWM) or “cross-talk” in wavelength multiplexed systems. Nevertheless, RZ format presents the advantage of being little affected by self-phase modulation (SPM) comparatively to a non-return-to-zero (NRZ) format. It often happens that the SPM induced by optical non-linearities in a line fiber gives rise to optical signal distortion that reduces the range and the capacity of optical transmission systems. In addition, RZ signals are suitable for being regenerated by synchronous modulation.
Conversely, the power spectrum density of a NRZ optical signal is narrower than that of an RZ signal. However, in NRZ format, both capacity and transmission range are limited by SPM. Furthermore, there are no optical or electronic regenerators in existence that are capable of processing such signals at high bit rates. In addition, the means for receiving such signals are unsuitable for integration and often introduce losses because of the interaction between successive “0” and “1” bits, and/or distortion, so that the extinction ratio of the signal after electrical filtering is degraded.
There also exist carrier-suppressed return-to-zero (CS-RZ) optical signals having the feature of presenting bits that are always phase-shifted by 180° relative to adjacent bits.
CS-RZ signals possess numerous advantages over conventional signals, both RZ and NRZ.
In the article entitled “320 Gbit/s (840 Gbit/s) WDM transmission over 367 km with 120 km repeater spacing using carrier-suppressed return-to-zero format”, published in Elec. Letters, Vol. 35, No. 31, Nov. 11, 1999, Y. Miyamoto et al. disclose experiments performed on a wavelength division multiplex (WDM) transmission line in which CS-RZ optical signals were transmitted over eight channels at 40 gibabits per second (Gbit/s). The transmission line used comprised both monomode fibers and fibers with inverse dispersion so as to obtain zero mean total dispersion. The experiments showed firstly that CS-RZ signals are tolerant of optical non-linearities. They also showed that CS-RZ signals at 40 Gbit/s provide a power level per transmission channel that is greater than that of a conventional RZ signal and that they present a power of spectrum density per channel that is narrower than that of conventional RZ signals at 40 Gbit/s, allowing WDM channels to be closer to each other.
Signals with such spectrum efficiency making it possible to occupy transmission bands more densely and/or to increase per-channel capacity are thus advantageous for future dense wavelength division multiplexing (DWDM) systems having total desired capacity in excess of petabits per second (Pbit/s).
Furthermore, in another article entitled “40 Gbit/s L-band transmission experiment using SBP-tolerant carrier-suppressed RZ format”, published in Elec. Letters, Vol. 35, No. 25, Dec. 9, 1999, A. Hirano et al. describe using a shifted dispersion optical fiber link in particular to compare the optimum dispersion stabilities of RZ, CS-RZ, and NRZ signals in the large (L) transmission band at frequencies in the range 1570 nanometers (nm) to 1605 nm, and they conducted their experiment up to high injected optical power levels. From those articles, it appears that CS-RZ signals at 40 Gbit/s present the most stable optimum dispersion and remain the closest to a total dispersion in the vicinity of 0 picosecond per nanometer (ps/nm). Dispersion tolerance is explained in particular by the phase inversion between adjacent bits which eliminates all inter-bit interference. Furthermore, CS-RZ signals subject the sensitivity of the receiver to little degradation at high power. Those results also confirm that CS-RZ signals are less sensitive to SPM than are NRZ signals.
In the second article, the generator producing the CS-RZ optical signals at 40 Gbit/s comprise a Mach-Zehnder modulator in push-pull mode fed with a sinusoidal electrical signal at a frequency of 20 gigahertz (GHz) and operating at the zero bias point for its transfer function.
Another type of CS-RZ clock signal generator is based on using a phase modulator to change the phase of each successive bit.
Because of their limited passbands, those prior art generators do not make it possible, at present, to produce stable CS-RZ signals at a modulation frequency in excess of 40 Gbit/s. In other words, such generators are unsuitable for producing CS-RZ signals at very high bit rates.